1. Field of the Invention
The present invention relates to a vehicle running control system, particularly to a vehicle running control system capable of easily and reliably altering the control mode or set points.
2. Description of the Related Art
For example, a conventional vehicle running control system is disclosed in Japanese Patent Laid-Open No. 58-39311. FIG. 19 is a schematic diagram illustrating a configuration of such a control system. In this figure, a set switch 1 is operated by a driver to issue an instruction indicating that a constant-speed running control process should be started. A cancel switch 2 is operated by a driver via a brake (not shown) to cancel the constant-speed running mode. A speed sensor 3 detects the running speed of a vehicle (not shown) and outputs a pulse train signal having a frequency proportional to the running speed. A main switch 5 acts as a power switch for supplying electric power from a battery 4 installed in the vehicle to a controller 6. The controller 6 gets in operation when the main switch 5 is turned on. The controller 6 includes a processor 6a such as a microcomputer for performing automatic control such that the running speed of the vehicle is maintained at a target speed V. According to input signals provided by the set switch 1, cancel switch 2 and speed sensor 3, the, controller 6 performs various processing, and outputs various control signals to a motor-driven throttle actuator 7 thereby performing automatic control. The throttle actuator 7 is disposed in an air inlet pipe 8 to control an opening (i.e., degrees of opening) of a throttle valve according to the control signal provided by the controller 6. The throttle actuator 7 controls the extent of opening of the throttle valve 9 in response to the operation of an accelerator pedal (not shown). The throttle actuator 7 is connected to the controller 6 so that it can also receive various control signals from the controller 6.
In addition to the functions provided in the conventional vehicle running control system shown in FIG. 19, another known vehicle running control system also has a vehicle-to-vehicle distance sensor (not shown) which detects a distance from a vehicle equipped with the distance sensor to a preceding vehicle running ahead, and performs automatic control based on the detected vehicle-to-vehicle distance to maintain the actual vehicle-to-vehicle distance at a target value.
In the vehicle running control system which performs automatic control on the opening of the throttle valve such that a vehicle runs at a target speed, there is provided a function for increasing or reducing the target speed without treading on an accelerator pedal. This function is accomplished by turning on manual switches called a resume/acceleration switch and a set/coast switch. When this function is in operation, the running speed is increased or decreased gradually independent of the target speed, and the target speed stored in memory is updated to a current running speed from moment to moment.
FIG. 20 is a time chart illustrating a gradual increase in the current running speed V1 during a time period in which the resume/acceleration switch is in an on-state. In FIG. 20, when the resume/acceleration switch is turned off, the running speed V2 at that time is stored in memory as the target speed. After that time, feedback control is done employing the running speed V2 as the target speed. However, the running speed overshoots up to V3 due to mechanical inertia. FIG. 21 is a time chart illustrating a gradual decrease in the current running speed V4 during a time period in which the set/coast switch is in an on-state. In FIG. 21, the running speed is V5 when the set/coast switch is turned off. However, the running speed undershoots to V6 due to mechanical inertia.
In the conventional vehicle running control system, as described above, to change the running speed by increasing or decreasing the target speed without treading on an accelerator pedal, it is required to turn on the manual switches called a resume/acceleration switch and set/coast switch. To do that, it is required to select correct switches from a plurality of switches disposed at various locations in a vehicle. Thus, this action distracts driver's attention from the view ahead, though for a rather short time. Furthermore, as shown in FIGS. 20 and 21, when the resume/acceleration switch or the set/coast switch is turned off, the change in the running speed cannot stop immediately due to mechanical inertia, and thus overshooting or undershooting occurs before the speed settles down to the running speed stored in the memory. Therefore, this type of conventional vehicle running control system gives the driver feeling of inconvenience, danger, unpleasantness, and shock.